Don't overwrite user setting of list_margin.

Closes #789

autoload/vimwiki/vars.vim

@@ -321,6 +321,7 @@ function! s:normalize_global_settings() abort
 endfunction
 
 
+let s:margin_set_by_user = 0
 function! s:populate_wikilocal_options() abort
   let default_values = {
         \ 'auto_diary_index': {'type': type(0), 'default': 0, 'min': 0, 'max': 1},
@@ -375,6 +376,9 @@ function! s:populate_wikilocal_options() abort
       for key in keys(default_values)
         if has_key(users_wiki_settings, key)
           call s:check_users_value(key, users_wiki_settings[key], default_values[key], 0)
+          if key ==# 'list_margin'
+            let s:margin_set_by_user = 1
+          endif
           let new_wiki_settings[key] = users_wiki_settings[key]
         else
           let new_wiki_settings[key] = default_wiki_settings[key]
@@ -498,11 +502,9 @@ function! s:normalize_wikilocal_settings() abort
       let wiki_settings.syntax = 'media'
     endif
 
-    if wiki_settings.syntax ==# 'markdown'
+    if wiki_settings.syntax ==# 'markdown' && !s:margin_set_by_user
       " default list margin to 0
       let wiki_settings.list_margin = 0
-    else
-      let wiki_settings.list_margin = -1
     endif
 
   endfor

test/list_margin.vader

@@ -21,6 +21,7 @@ Execute (Set syntax default):
   set shiftwidth=8
   AssertEqual 8, &shiftwidth
   call SetSyntax('default')
+  call vimwiki#vars#set_wikilocal('list_margin', -1, vimwiki#vars#get_bufferlocal('wiki_nr'))
 
 Execute (Generate Links):
   VimwikiGenerateLinks
@@ -47,6 +48,7 @@ Expect (Links with margin == 2):
 
 Execute (Set syntax media):
   call SetSyntax('media')
+  call vimwiki#vars#set_wikilocal('list_margin', -1, vimwiki#vars#get_bufferlocal('wiki_nr'))
 
 Execute (Generate Links):
   VimwikiGenerateLinks
@@ -73,6 +75,7 @@ Expect (Links with margin == 1):
 
 Execute (Set syntax markdown):
   call SetSyntax('markdown')
+  " list margin should default to 0 for markdown
 
 Execute (Generate Links):
   VimwikiGenerateLinks